As the processes for producing riboflavin by fermentation, processes characterized by culturing mutant strains of Eremothecium ashbyii, Ashbya gossypii, Candida flareri, Saccharomyces cerevisiae, etc. are known (Progress Industrial Microbiology, 1, 139 (1959), U.S. Pat. No. 5,164,303 and Japanese Published Unexamined Patent Application No.112,996/88).
In addition, European Publication No. 405,370 describes that genetically engineered microorganisms belonging to the genus Bacillus in which Bacillus-derived genes encoding enzymes involved in the biosynthesis of riboflavin are introduced, are used for riboflavin production. The publication contains a description of Bacillus-derived genes encoding enzymes involved in the biosynthesis of riboflavin, but there is no specific description of a DNA which is derived from a microorganism belonging to the genus Corynebacterium or Brevibacterium and which, upon introduction into a riboflavin-requiring microorganism, confers on the microorganism the ability to complement its riboflavin requirement.